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Arrays

Anarray is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. You have seen an example of arrays already, in themain method of the "Hello World!" application. This section discusses arrays in greater detail.

An array of 10 elements.

Each item in an array is called anelement, and each element is accessed by its numericalindex. As shown in the preceding illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.

The following program,ArrayDemo, creates an array of integers, puts some values in the array, and prints each value to standard output.

class ArrayDemo {    public static void main(String[] args) {        // declares an array of integers        int[] anArray;        // allocates memory for 10 integers        anArray = new int[10];                   // initialize first element        anArray[0] = 100;        // initialize second element        anArray[1] = 200;        // and so forth        anArray[2] = 300;        anArray[3] = 400;        anArray[4] = 500;        anArray[5] = 600;        anArray[6] = 700;        anArray[7] = 800;        anArray[8] = 900;        anArray[9] = 1000;        System.out.println("Element at index 0: "                           + anArray[0]);        System.out.println("Element at index 1: "                           + anArray[1]);        System.out.println("Element at index 2: "                           + anArray[2]);        System.out.println("Element at index 3: "                           + anArray[3]);        System.out.println("Element at index 4: "                           + anArray[4]);        System.out.println("Element at index 5: "                           + anArray[5]);        System.out.println("Element at index 6: "                           + anArray[6]);        System.out.println("Element at index 7: "                           + anArray[7]);        System.out.println("Element at index 8: "                           + anArray[8]);        System.out.println("Element at index 9: "                           + anArray[9]);    }}

The output from this program is:

Element at index 0: 100Element at index 1: 200Element at index 2: 300Element at index 3: 400Element at index 4: 500Element at index 5: 600Element at index 6: 700Element at index 7: 800Element at index 8: 900Element at index 9: 1000

In a real-world programming situation, you would probably use one of the supportedlooping constructs to iterate through each element of the array, rather than write each line individually as in the preceding example. However, the example clearly illustrates the array syntax. You will learn about the various looping constructs (for,while, anddo-while) in theControl Flow section.

Declaring a Variable to Refer to an Array

The preceding program declares an array (namedanArray) with the following line of code:

// declares an array of integersint[] anArray;

Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written astype[], wheretype is the data type of the contained elements; the brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in thenaming section. As with variables of other types, the declaration does not actually create an array; it simply tells the compiler that this variable will hold an array of the specified type.

Similarly, you can declare arrays of other types:

byte[] anArrayOfBytes;short[] anArrayOfShorts;long[] anArrayOfLongs;float[] anArrayOfFloats;double[] anArrayOfDoubles;boolean[] anArrayOfBooleans;char[] anArrayOfChars;String[] anArrayOfStrings;

You can also place the brackets after the array's name:

// this form is discouragedfloat anArrayOfFloats[];

However, convention discourages this form; the brackets identify the array type and should appear with the type designation.

Creating, Initializing, and Accessing an Array

One way to create an array is with thenew operator. The next statement in theArrayDemo program allocates an array with enough memory for 10 integer elements and assigns the array to theanArray variable.

// create an array of integersanArray = new int[10];

If this statement is missing, then the compiler prints an error like the following, and compilation fails:

ArrayDemo.java:4: Variable anArray may not have been initialized.

The next few lines assign values to each element of the array:

anArray[0] = 100; // initialize first elementanArray[1] = 200; // initialize second elementanArray[2] = 300; // and so forth

Each array element is accessed by its numerical index:

System.out.println("Element 1 at index 0: " + anArray[0]);System.out.println("Element 2 at index 1: " + anArray[1]);System.out.println("Element 3 at index 2: " + anArray[2]);

Alternatively, you can use the shortcut syntax to create and initialize an array:

int[] anArray = {     100, 200, 300,    400, 500, 600,     700, 800, 900, 1000};

Here the length of the array is determined by the number of values provided between braces and separated by commas.

You can also declare an array of arrays (also known as amultidimensional array) by using two or more sets of brackets, such asString[][] names. Each element, therefore, must be accessed by a corresponding number of index values.

In the Java programming language, a multidimensional array is an array whose components are themselves arrays. This is unlike arrays in C or Fortran. A consequence of this is that the rows are allowed to vary in length, as shown in the followingMultiDimArrayDemo program:

class MultiDimArrayDemo {    public static void main(String[] args) {        String[][] names = {            {"Mr. ", "Mrs. ", "Ms. "},            {"Smith", "Jones"}        };        // Mr. Smith        System.out.println(names[0][0] + names[1][0]);        // Ms. Jones        System.out.println(names[0][2] + names[1][1]);    }}

The output from this program is:

Mr. SmithMs. Jones

Finally, you can use the built-inlength property to determine the size of any array. The following code prints the array's size to standard output:

 System.out.println(anArray.length);

Copying Arrays

TheSystem class has anarraycopy method that you can use to efficiently copy data from one array into another:

public static void arraycopy(Object src, int srcPos,                             Object dest, int destPos, int length)

The twoObject arguments specify the array to copyfrom and the array to copyto. The threeint arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.

The following program,ArrayCopyDemo, declares an array ofString elements. It uses theSystem.arraycopy method to copy a subsequence of array components into a second array:

 class ArrayCopyDemo {    public static void main(String[] args) {        String[] copyFrom = {            "Affogato", "Americano", "Cappuccino", "Corretto", "Cortado",               "Doppio", "Espresso", "Frappucino", "Freddo", "Lungo", "Macchiato",                  "Marocchino", "Ristretto" };                String[] copyTo = new String[7];        System.arraycopy(copyFrom, 2, copyTo, 0, 7);        for (String coffee : copyTo) {            System.out.print(coffee + " ");                   }    }}

The output from this program is:

Cappuccino Corretto Cortado Doppio Espresso Frappucino Freddo

Array Manipulations

Arrays are a powerful and useful concept used in programming. Java SE provides methods to perform some of the most common manipulations related to arrays. For instance, theArrayCopyDemo example uses thearraycopy method of theSystem class instead of manually iterating through the elements of the source array and placing each one into the destination array. This is performed behind the scenes, enabling the developer to use just one line of code to call the method.

For your convenience, Java SE provides several methods for performing array manipulations (common tasks, such as copying, sorting and searching arrays) in thejava.util.Arrays class. For instance, the previous example can be modified to use thecopyOfRange method of thejava.util.Arrays class, as you can see in theArrayCopyOfDemo example. The difference is that using thecopyOfRange method does not require you to create the destination array before calling the method, because the destination array is returned by the method:

class ArrayCopyOfDemo {    public static void main(String[] args) {        String[] copyFrom = {            "Affogato", "Americano", "Cappuccino", "Corretto", "Cortado",               "Doppio", "Espresso", "Frappucino", "Freddo", "Lungo", "Macchiato",                  "Marocchino", "Ristretto" };                String[] copyTo = java.util.Arrays.copyOfRange(copyFrom, 2, 9);                for (String coffee : copyTo) {            System.out.print(coffee + " ");                   }                }}

As you can see, the output from this program is the same, although it requires fewer lines of code. Note that the second parameter of thecopyOfRange method is the initial index of the range to be copied, inclusively, while the third parameter is the final index of the range to be copied,exclusively. In this example, the range to be copied does not include the array element at index 9 (which contains the stringLungo).

Some other useful operations provided by methods in thejava.util.Arrays class are:

• Searching an array for a specific value to get the index at which it isplaced (thebinarySearch method).

• Comparing two arrays to determine if they are equal or not (theequals method).

• Filling an array to place a specific value at each index (thefill method).

• Sorting an array into ascending order. This can be done eithersequentially, using thesort method, or concurrently, using theparallelSort method introduced in Java SE 8. Parallel sorting oflarge arrays on multiprocessor systems is faster than sequential array sorting.

• Creating a stream that uses an array as its source (thestreammethod). For example, the following statement prints the contents of thecopyTo array in the same way as in the previous example:

  java.util.Arrays.stream(copyTo).map(coffee -> coffee + " ").forEach(System.out::print);

  SeeAggregate Operations for more information about streams.

• Converting an array to a string. ThetoString methodconverts each element of the array to a string, separates them with commas, thensurrounds them with brackets. For example, the following statement converts thecopyTo array to a string and prints it:

  System.out.println(java.util.Arrays.toString(copyTo));

  This statement prints the following:

  [Cappuccino, Corretto, Cortado, Doppio, Espresso, Frappucino, Freddo]